The present application relates in general to communication systems transferring data between a host and a device using differential signals traveling through a differential pair of conductors in a cable assembly. Common-mode noise signals, from a variety of potential sources, may add to a desired differential data signal, exit the cable assembly and enter the device where it can degrade the quality not only of the underlying differential data signal carried on the differential pair of conductors, but also of other signals in close proximity.
In the context of this document, data may be analog or digital or, more generally data may be represented by continuous or discrete levels, and at continuous or discrete times. Some data may be transferred according to a high-speed data protocol. Common-mode noise signals may include random (stochastic) and non-random signals that may be caused by a variety of physical processes.
As a differential signal travels over the cable assembly's differential pair of conductors, an undesired phenomenon called “mode conversion” can occur whereby some of the differential signal is transformed into a common-mode signal component. Mode conversion is due to less than ideal isolation between two conductors of the differential pair and practical manufacturing constraints leading to the two conductors not being perfectly matched. These practical limitations are even more pronounced with thinner gauge conductors, which are used to produce thin, light cable assemblies. Two example applications where cable weight and size are of paramount importance are virtual reality and augmented reality headset devices. Unfortunately, these are also applications that demand high resolution video, high-quality audio, and that carry a multitude of other data signals, both low-speed and high-speed.
In this situation, a common-mode signal component can arise on the conductors of the differential pair when it is in the presence of electromagnetic interference (EMI). The source of the interference may be due to electromagnetic coupling from a single-ended signal conductor, electromagnetic coupling from another differential conductor pair, or from a source external to the cable assembly.
The problem with the common-mode signal is threefold. First, a differential receiver at the receiving end of the cable assembly is expecting a differential signal, and the effect of the common-mode component can degrade the quality of the reception, possibly resulting in errors. In particular, each input of a differential receiver is typically set at an optimized bias voltage around which the differential signal fluctuates. But the addition of a common-mode signal component perturbs the receiver input from this optimized point—potentially resulting in poorer reception performance.
Another negative impact of the common-mode signal component is that it will return through the ground terminals between the device and the cable. In many applications, the device has a floating ground, i.e. its only connection to ground is through the cable assembly, so imparting the common-mode signal into the local device ground will perturb the ground seen by other circuits on the device. Some of these circuits may be particularly sensitive to ground noise. For example, in display drivers used in headsets the ground noise may cause speckling, blurring, color bleeding, or other visual distortions.
A third drawback of the common-mode signal component is that it is more prone to emit EMI and cause interference for other signals. Differential signals are used because the radiation emitted by each of the two conductors (in the differential pair) tends to cancel the other one out, resulting in lower overall EMI. However, if there is a common-mode component on the differential conductor pair, then not only will the two fields not cancel; they will in fact tend to constructively add resulting in higher emitted radiation. This EMI can then impinge on other conductors nearby, and single-ended conductors will be particularly vulnerable to picking up this EMI. The result can be errors in the reception of low-bandwidth signals that may be carried on the single-ended conductors.
FIG. 1 illustrates components in a conventional data link that includes a cable assembly. Communication system 100 comprises one terminal 101 called a host which drives at least one signal to another terminal 102 called a device. The signal is carried from host 101 to device 102 by way of a cable assembly 103, which may carry other signals as well. Cable assembly 103 comprises at least one differential conductor pair to reduce a common-mode signal component delivered to a differential receiver inside device 103.
FIG. 2 illustrates further detail of components in the cable assembly 103. It may include one or more differential signal pairs of conductors (a first differential pair is illustrated as 104a-b) and optionally one or more single-ended conductors (a first single-ended conductor is illustrated as 105). High-bandwidth data is conventionally carried over differential pairs of conductors. This may include content such as video and audio (e.g., according to a High-Definition Multimedia Interface (HDMI), Mobile High-Definition Link (MHL), or DisplayPort standard). It can also include carrying more generic data for example according to a protocol such as, but not limited to, Universal Serial Bus (USB) or Peripheral Component Interconnect Express (PCI Express), where data can flow in each direction.
A cable assembly such as 103 in many cases comprises one or more single-ended conductors used to carry power or lower bandwidth signals. This can include a power supply for the device; a ground return; control signals such as Inter-Integrated Circuit (I2C), universal asynchronous receiver/transmitter (UART), or Serial Peripheral Interface (SPI) signals; analog audio signals; digital audio signals such as Inter-IC Sound (I2S), Sony/Philips Digital Interface Format (S/PDIF), pulse-density modulation (PDM), or pulse-code modulation (PCM); and sensor data. These low-bandwidth signals may be sourced by either the host or device depending on the system configuration.
FIG. 3 illustrates common-mode related impairments in a conventional link to a device. A traditional communication system 300 includes host 301, device 302, and cable assembly 303. The cable assembly 303 includes differential pair of conductors 304a-b, a single-ended conductor 305, and ground conductor 306. Host 301 includes driver circuit 310a for the single-ended conductor 305, and differential driver circuit 310b for differential pair of conductors 304a-b. Device 302 includes receive circuit 320a for signals on single-ended conductor 305, and differential receiver 320b for differential signals on differential pair of conductors 304a-b. The system is perturbed by common-mode signal 330.
Differential receiver 320b expects a differential signal and the effect of the common-mode component 330 can degrade the quality of the reception, possibly resulting in errors. In particular, each input of differential receiver 320b is typically set at an optimized bias voltage around which the differential signal fluctuates. But the addition of a common-mode signal component perturbs the receiver 320b input from this optimized point potentially resulting in poorer reception performance.
Common-mode signal component 330 will return via device 302 and differential receiver 340 through ground conductor 306. In many applications, the device 302 has a floating ground, i.e. its only connection to ground is through the cable 303, so imparting the common-mode signal into the local device ground will perturb the ground seen by other circuits on the device 302. Some of these circuits may be particularly sensitive to ground noise. For example, in display drivers used in headsets the ground noise may cause speckling, blurring, color bleeding, or other visual distortions in the display.
A further drawback of the common-mode signal component 330 is that it is more prone to emit EMI and cause interference for other signals. Differential signals are used because the radiation emitted by each conductor 304a-b in the differential pair tends to cancel the other out resulting in lower overall EMI. However, if there is a common-mode component 330 on the differential pair of conductors 304a-b, then the two fields will not cancel; they will in fact tend to constructively add resulting in higher emitted radiation 340. This EMI 340 can then impinge on other conductors nearby, and single-ended conductors 305 will be particularly vulnerable to picking up this EMI. The result can be errors in the reception of the low-bandwidth signals carried on the single-ended conductors 305.
One example, among many, of such a communication system is a headset or visual reality goggle linked to an audiovisual device using a high-speed data cable, for example according to an HDMI, MHL, or DisplayPort standard, or another standard for the transfer of high-speed audiovisual data. Other examples of such systems include USB and PCI Express.